If light from a beam source is coupled into one end of a light guide, the beam profile at the other end of the light guide is determined not only by the beam source but also by the homogenisation properties of the light guide. The near field power distribution desired for a laser processing machine at the output end of the light guide can take the shape (in a radial direction) of, for example, a mountain peak (conical tip with marked intensity maximum at the beam centre) or a flat-top mountain peak, (cylindrical shape, i.e. plateau of substantially constant intensity, “top-hat”). By setting a beam profile with a marked intensity maximum at the beam centre, the welding penetration depth in a laser welding process can be increased. By setting a top-hat profile on the other hand, a wide seam and hence a larger joint width between the two workpieces to be welded can be achieved.
US 2010/0150201 A1 describes the coupling of laser radiation of a fibre bundle into a single fibre of a fibre optic cable. For better alignment of the fibres relative to the radiation entering therein, a mode mixer is used, which comprises a means for altering the curvature of the fibre optic cable. It is intended to achieve a homogenisation of the energy density of the laser beam by winding the fibre optic cable in loops.
KR 10-2010-0068694 (Kim Jom Sool) discloses a device in which a section of a fibre optic cable is bent in order to achieve a uniform brightness distribution of a laser beam guided in the fibre optic cable. For that purpose the fibre optic cable is fixed at two points to a fixing plate and a movement device arranged therebetween displaces the section of the fibre optic cable running between said two points in order to adjust the curvature of the fibre optic cable.
In order to protect light guides against damage from external influences, they are surrounded inter alia by a protective tube and form a fibre optic cable. The protective tube serves to withstand tensile, bending and transverse compressive stresses of the fibre optic cable. In particular when the light guide is used for transporting a high power laser beam, the protective tube should also ensure that no forces leading to a distortion or deformation of the light guide are able to act on the light guide. Such a protective tube is known, for example, from DE 20 2004 004 817 U1. The protective tube described therein has a high stiffness so that it can be bent only as far as a minimal bend radius of 150 mm, at which no damaging forces act on the light guide with the light guide diameters used there.